1. NMR: Theory and Equivalence

2. Nuclear Magnetic Resonance
Powerful analysis
Identity
Purity
No authentic needed
Analyze nuclei
1H, 13C, 31P, etc
Get information of how they are attached

3. Nuclear Magnetic Resonance
Based on nuclear spin
Magnetic
Large electromagnets create energy differences in nuclei
Resonance
Signal based on absorbed light in resonance with energy difference

5. Nuclear Spin Some nuclei have spin
1H, 13C
No spin if nuclei have even number of protons and neutrons (12C)
Nuclei align spin with and against magnetic field

6. External Magnetic Field
Bigger energy difference between spin states
For absorption, Ephoton = Espin gap
Spin flip occurs
Consider the proton NMR spectrum of CH4

7. NMR Spectrum CH4 in magnet Protons line up with/against RF energy
Only RF energy = gap energy absorbed
Spectrum shows an absorption

8. Magnetic Field Experienced
Not all nuclei in a molecule experience the same magnetic field, even if they are in the same magnet
Why? Shielding!
Consider CH3Cl

9. NMR Spectrum CH3Cl in magnet Electronegative Cl deshield protons
Protons experience bigger external field
Only RF energy = gap energy absorbed
Spectrum shows a higher frequency absorption

10. Other Sources of Shielding
Anisotropy
Different effect along different axis
Can have dramatic shielding/ deshielding effects

11. NMR Spectrum Need standard: TMS (shielded)
Need relative frequency: ppm
ppm = Hz shift/ MHz magnet
60 mHz
200 MHz

12. Four Questions How many signals? Equivalence
Where on spectrum? Chemical Shift
How big? Integration
Shape? Splitting (coupling)

13. Number of Signals
Four sets of protons in different chemical environments
Four sets of equivalent protons
Leads to four distinct signals

14. Equivalence Experience same average magnetic field due to
Free rotation
Plane of symmetry
Axis of symmetry

15. How many 13C signals?

16. How many 1H signals?

17. A more complicated problem